Electrocardiograms (ECG or EKG) are commonly used to monitor and diagnose heart conditions. An ECG is a recording made by an electrocardiograph of electric waves generated during activity in the heart. To measure an ECG, electrodes, or electrical contacts, are placed on the skin. With each heart beat, an electrical impulse or wave travels through the heart, which, in turn, causes the heart muscles to pump blood from the heart to the rest of the body. The ECG records the voltage difference between pairs of electrodes and displays the rhythm of the heart and weaknesses that may be present in different parts of the heart.
In electrocardiography, a “lead” refers to a pair of electrodes that form an imaginary line in a patient along which electrical signals are measured. Three external leads, known as limb leads, form the three legs of what is known in the art as Einthoven's triangle. Einthoven's triangle defines an imaginary equilateral triangle having the heart at its center. Lead I is defined by a vector extending from a negative electrode on the right arm to a positive electrode on the left arm. Lead II is defined by a vector extending from the negative electrode on the right arm to a positive electrode on the left leg. Lead III is defined by a vector extending from a negative electrode on the left arm to the positive electrode on the left leg.
Current implantable cardiac rhythm management devices typically detect cardiac electrical activity between electrodes in or around the heart and the pulse generator, or between two electrodes on the pulse generator. The Reveal® Plus by Medtronic is a subcutaneously implantable loop recorder that can measure and record ECG data. U.S. Pat. No. 5,313,953 by Yomtov et al. discloses an implantable cardiac monitor with a subcutaneous lead proximate the heart, which is designed to process ECG signals. Additionally, prior art pacemaker and defibrillator manufacturers use associated cardiac leads connected to a sense amplifier in implantable pulse generators to facilitate ECG inputs.
Prior art implantable therapy devices such as pacemakers, defibrillators, and other devices have also been used to monitor thoracic impedance to detect respiration-related conditions, such as “lung water” and minute ventilation. This has been accomplished with the use of typically a single electrode implanted within the thoracic cavity, and an implantable pulse generator. While these implantable transthoracic methods of monitoring respiration were sometimes sufficient for diagnostic purposes, their single vector measurement approach leaves room for improvement for monitoring respiration for the purpose of timing the delivery of a therapy.
Significantly, however, cardiac electrical activity occurs over multiple electrical planes and vectors. Typically, approaches used by the prior art to monitor cardiac electrical activity lack complete information because they only detect electrical activity along one vector, such as between an electrode on a cardiac lead and the pulse generator. Furthermore, the prior art does not permit monitoring multiple ECG leads with an implantable medical device and integrating multiple-lead ECG information by an implantable medical device.
Thus, there remains a need in the art to be able to monitor cardiac electrical activity with an implantable medical device that can detect the ECG along at least three vectors, providing more complete information about heart function than existing devices and permitting discrimination of pathological events that cannot be detected with single-vector devices. Additionally, there exists a need in the art for a method for measuring ECGs and analyzing cardiac output that is also capable of detecting a patient's rate of respiration.